188 THE ARTICULATIONS OF THE HORSE 



part is attached. The remainder passes obUquely downward and forward to the 

 anterior surface of the first phalanx, where it joins the extensor tendon. This 

 ligament possesses considerable elasticity, and is the highly modified interosseous 

 medius muscle. It consists mainly of tendinous tissue, but contains a variable 

 amount of striped muscular tissue, especially in its deep part and in young subjects. 

 Its principal function is to support the fetlock, i. e., to prevent excessive dorsal 

 flexion of the joint when the weight is put on the limb. The branches which join 

 the common extensor tendon limit volar flexion of the interphalangeal joints in 

 certain phases of movement. 



The inferior sesamoidean ligaments are three in number — superficial, middle, 

 and deep. The superficial or straight ligament (Ligamentum sesamoideum rec- 

 tum) is a fiat band and is somewhat wider above than below. ^ It is attached above 

 to the bases of the sesamoid bones and the intersesamoid ligament, below to the 

 complementary fibro-cartilage of the proximal end of the second phalanx. The 

 middle ligament is triangular, with thick, rounded margins (Ligamenta ol)liciua) and 

 a thin central portion.- Its base is attached to the sesamoid bones and intersesa- 

 moid ligament, and its deep face to the triangular rough area on the posterior 

 surface of the first phalanx. The deep or cruciate ligament (Ligamenta sesamoidea 

 cruciata) consists of two thin layers of fibers which arise on the base of the sesa- 

 moid bones, cross each other, and end on the opposite eminence on the proximal 

 end of the first phalanx. 



The short sesamoidean ligaments (Ligamenta sesamoidea brevia) are best 

 seen by opening the joint in front and pushing the sesamoid bones backward; they 

 are covered by the synovial membrane. Each is a short band which extends from 

 the anterior part of the ])ase of the sesamoid bone outward to the posterior margin 

 of the articular surface of the first phalanx. 



The inferior sesamoidean ligaments may be regarded as digital continuations 

 of the suspensory ligament, the sesamoid bones being intercalated in this remark- 

 able stay apparatus by which the fetlock is supported and concussion diminished. 



THE PASTERN JOINT 



This, the proximal interphalangeal articulation (Articulatio phalangis se- 

 cundse), is a ginglymus formed by the junction of the distal end of the first phalanx 

 and the proximal end of the second phalanx. 



The articular surfaces are: (1) On the first phalanx, two slightly unequal con- 

 vex areas with an intermediate shallow groove; (2) on the second phalanx, a cor- 

 responding surface, completed behind by a plate of fibro-cartilage. 



The joint capsule is close-fitting in front and laterally, where it blends with the 

 extensor tendon and the lateral ligaments respectively. Behind it pouches upward 

 a little and is reinforced by the straight sesamoidean ligament and the branches of 

 the superficial flexor tendon. 



There are two lateral and four volar ligaments. 



The lateral ligaments, internal and external (Ligamentum collaterale radiale, 

 ulnare) are very short and strong bands which are attached alcove on the emi- 

 nence and depression on each side of the distal end of the first phalanx, and below 

 on the eminence on either side of the proximal end of the second phalanx. The 

 direction of the ligaments is about vertical and, therefore, does not correspond to 

 the digital axis. 



The volar or posterior ligaments consist of central and lateral pairs of bands 

 which are attached below to the i:)osterior margin of the proximal end of the second 



^This is often called the Y-shnped ligament — a rather undesirable name, since it is not 

 bifurcate. 



^It is also called the V-shaped ligament. 



